Optical storage media are media in which data are stored in an optically readable manner, for example by means of a pickup comprising a laser for illuminating the optical storage medium and a photo-detector for detecting the reflected light of the laser beam when reading the data. In the meanwhile a large variety of optical storage media are available, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to about 50 Gigabyte (GB). Digital data are stored in these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc, which allows to store up to 50 GB on a dual layer disc. Available formats are at present for example read-only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-Ray disc, a pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2 T to 8 T, maximum 9 T, is used, where T is the channel bit length, which corresponds with a minimum mark length of 138-160 nm.
The diffraction limit of optical instruments as described by the Abbe theory is about lambda/2 NA, which is 238 nm for a Blu-Ray type pickup with a wavelength lambda=405 nm and a numerical aperture NA=0.85. This theoretical minimal detectable length from the diffraction theory is corresponding to a period of the pattern function, which is formed of a pit and of a land having the same length. The smallest detectable element of such a system is a pit or a land having a length of about lambda/4 NA, which corresponds for a Blu-Ray type pickup with a length of 120 nm.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by including a nonlinear layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The nonlinear layer can be understood as a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the optical resolution limit of lambda/4 NA of a corresponding optical pickup. Super-resolution optical discs comprising a super-resolution near-field structure are known for example from WO 2005/081242, US 2004/0257968 and WO 2004/032123.
The super-resolution effect allows increasing the resolution of the pickup for reading of the marks on an optical disc, but does not allow reducing the track pitch.
A pickup providing three light beams with a main beam and two satellite beams for providing a tracking signal for reading data from an optical storage medium is known since the introduction of the CD. In these arrangements, the light intensity of the satellite beams is much smaller than the intensity of the main beam. An apparatus of this kind is described for example in U.S. Pat. No. 6,137,758, which uses a detector unit with a main detector for providing a data signal and a pair of satellite detectors for detecting the satellite beams as reflected from the optical storage medium for providing a tracking error signal. The two satellite detectors allow distinguishing between a CD and a DVD in addition to the generation of a tracking signal. The apparatus is in particular designed for optical discs having a track pitch between adjacent tracks, which is below the diffraction limit of the pickup. An apparatus comprising a pickup providing a main beam and two satellite beams for reading data from a super-resolution optical disc is known for example from EP-B-2009627.
WO 2006/004338 describes an apparatus comprising a pickup for reading data from a super-resolution optical disc, wherein the pickup provides a first beam having a light intensity being sufficient for providing a super-resolution effect and a second beam following the first beam having not the light intensity for providing the super-resolution effect. By taking into account a temporal delay between the reflected signal of the first beam and the reflected signal of the second beam, reflected light outside of a reproduction beam spot of the super-resolution area is excluded, thereby improving the reproduction signal characteristics of the HF-signal.
In the publication Wu and Chong, Applied Optics 36, 1997, pages 6668-6677, a method for reading data from a super-resolution optical disc is described, wherein an assistant beam with a modified beam profile is used to optimize the thermal aperture on the super-resolution optical disc.